The present invention relates to a process for preventing the evaporation of a liquefied gas, particularly liquid methane, stored in an impervious and thermally insulating tank which may or may not be built into a bearing structure of a ship, particularly a methane tanker, and to the device for implementing this process.
Liquid methane is generally stored in liquid form at a pressure close to atmospheric pressure and at a temperature of around xe2x88x92163xc2x0 C. In order to limit the evaporation of liquid methane during transport, it has already been proposed that the thermal insulation of the tank be improved, using various methods which are described in French patent applications 2 535 831, 2 586 082, 2 629 897 and 2 683 786, all of which are in the name of the Applicant Company. Improvements to the thermal insulation of the tank have made it possible to lower the nominal degree of evaporation per day of storage, from 0.30% to about 0.15%, but it is difficult to improve beyond this.
On a methane tanker, each tank is generally connected to a mast riser on the main deck of the ship, to allow the evaporated gas to escape, as this gas would otherwise generate an inadmissible overpressure in the tank. In order to avoid discharging the evaporated gas into the atmosphere, which gas constitutes pollutant emissions which are all the more unacceptable when the ship is near a port, and to avoid thus losing part of the cargo, it is known practice to use the evaporated gas to propel the ship. For this purpose, the engine room of the ship is generally equipped with a steam turbine which is adapted to run both on evaporated gas and on diesel or fuel oil. However, steam turbines have a low efficiency and the dual functionality of the turbine entails lengthening the engine room, and this means lengthening the ship or reducing the size of the storage tanks. Furthermore, with a degree of evaporation of the order of 0.15%, the evaporated gas supplies only 40 to 80% of the energy needed by the steam turbine, which therefore has to constantly run on the diesel or the fuel oil as well.
To avoid the drawback associated with burning the evaporated gases and to avoid consuming part of the cargo before it is offloaded, it has also been proposed that a reliquefaction plant be installed on the deck of the ship in order to reliquefy the evaporated gases and return them to the tank. However, this solution is extremely onerous to implement because the initial outlay for a reliquefaction plant is very high and the power requirement of a reliquefaction plant is also very high. Furthermore, as a cargo of methane is generally not pure, it is necessary to provide separate liquefaction of the various fractions of the cargo, which entails the use of separation columns, something which is difficult to manage on a ship subject to swell.
The object of the invention is to eliminate the aforementioned drawbacks and to provide a process for preventing the evaporation of a liquefied gas stored in an impervious and thermally insulating tank which may or may not be incorporated into a bearing structure of a ship and which is simple and economical to implement and to operate.
To this end, the subject of the invention is a process for preventing the evaporation of a liquefied gas stored in an impervious and thermally insulating tank built into a bearing structure of a ship or located in a set of floating or land-side storage tanks, characterized in that it consists in passing a fluid refrigerant through the mass of liquefied gas to cool said mass to a temperature slightly below its reference storage temperature, so as to compensate for the heating of said mass as a result of thermal leaks during transport or storage thereof. Thus, the evaporation of the liquefied gas is prevented, or, at the very least, limited. What happens is that if the liquefied gas begins to evaporate into the gaseous volume lying on top of the liquefied mass in the tank, the circulation of the fluid refrigerant will cause automatic reliquefaction of the evaporated gas, by heat transfer at the interface between the liquefied gas and the evaporated gas.
The invention is also aimed at a device for implementing the aforementioned process, characterized in that it comprises, for each tank, a heat exchanger immersed in the mass of liquefied gas that is to be cooled, a compressor for compressing the fluid refrigerant at the exit of the exchanger, and a refrigeration unit for cooling the compressed fluid refrigerant to its refrigeration temperature before it enters the heat exchanger.
Advantageously, the device comprises a unit for circulating sea water to cool the compressed fluid refrigerant before it enters the refrigeration unit. This sea water circulation unit may be connected to a ballast collector of a ship.
In a particular embodiment, the fluid refrigerant is in the liquid phase, is preferably liquid nitrogen, as it enters the heat exchanger and vaporizes as it passes through the mass of liquefied gas, the refrigerating unit being designed to reliquefy the fluid refrigerant each cycle. This alternative form is particularly efficient because the latent heat of the liquid refrigerant is used to refrigerate the cargo. Of course, the fluid refrigerant could be in the gaseous phase, and in this case the gaseous refrigerant undergoes a pressure reduction as it is heated in the heat exchanger, for example on the known Joule-Thomson cycle.
According to another feature, the refrigerating unit is designed to take the fluid refrigerant down to a refrigeration temperature which is approximately 30xc2x0 C. below the reference temperature for the mass of liquefied gas before it enter he heat exchanger.
According to yet another feature of the invention, each tank is fitted with a pressure gage for monitoring the variations in pressure in the volume of gas lying on top of the mass of liquefied gas in the tank. In this case, the pressure gage may initiate the circulation of the fluid refrigerant as soon as the pressure detected by the pressure gage exceeds a first predetermined pressure threshold value, for example 5 mmbar [sic] above the reference storage pressure, which is generally of the order of 1,060 mmbar [sic], and stops it as soon as the detected pressure is below a second determined pressure threshold value, for example 5 mmbar [sic] below said reference pressure.
Advantageously, the heat exchanger is supported inside the tank by a tower for loading/offloading the liquefied gas, which tower is provided on one of the vertical transverse walls of the tank.
The heat exchanger may comprise one or more hairpin tubes, the ends of which pass through the roof of the tank. In this case, each hairpin tube or group of hairpin tubes may be surrounded on its sides by a hollow pipe forming a convection well, open at its two vertical ends, to generate a convective movement in the mass of liquefied gas through each well.
Advantageously, the compressor and the refrigeration unit are installed on the deck of the ship, in line with the loading/offloading tower of each tank.
In order to give a better understanding of the subject of the invention, one embodiment thereof, depicted in the appended drawings, will now be described by way of purely illustrative and non-limiting example.